# ExistingProvider

**Kind:** Interface

**Source:** [`packages/common/interfaces/modules/provider.interface.ts`](https://github.com/nestjs/nest/blob/master/packages/common/interfaces/modules/provider.interface.ts#L157)

**Part of:** [Common](subsystem-packages-common)

Interface defining an *Existing* (aliased) type provider.

For example:
```typescript
const loggerAliasProvider = {
  provide: 'AliasedLoggerService',
  useExisting: LoggerService
};
```

`ExistingProvider` defines an alias for an already-registered dependency injection provider. Instead of creating a new instance, it maps a new `provide` token to the same instance represented by `useExisting`.

## Properties

| Property | Type |
|---|---|
| `provide` | `InjectionToken` |
| `useExisting` | `any` |

## Diagram

```mermaid
graph LR
  A[Injection Token: AliasedLoggerService] --> B[ExistingProvider]
  B -->|useExisting| C[Existing Provider: LoggerService]
  C --> D[Shared LoggerService Instance]
```

## Usage

```typescript
import { ExistingProvider } from '@nestjs/common';

class LoggerService {
  log(message: string) {
    console.log(message);
  }
}

const loggerAliasProvider: ExistingProvider = {
  provide: 'AliasedLoggerService',
  useExisting: LoggerService,
};

@Module({
  providers: [LoggerService, loggerAliasProvider],
})
export class AppModule {}
```

## AI Coding Instructions

- Use `useExisting` when multiple injection tokens must resolve to the exact same provider instance.
- Ensure the target referenced by `useExisting` is registered in the same module or available through imported/exported modules.
- Use a distinct `provide` token for the alias, such as a string, symbol, class, or custom injection token.
- Do not use `useExisting` when a separate instance is required; use `useClass`, `useFactory`, or `useValue` instead.
- Prefer aliases for backwards-compatible token migrations or interface-based dependency injection.

## Used by

3 references from 3 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (3)

- `InternalCoreModule` — `packages/core/injector/internal-core-module/internal-core-module.ts`:16
- `Module` — `packages/core/injector/module.ts`:44
- `DependenciesScanner` — `packages/core/scanner.ts`:75